Q: What is the Difference between Syndicated and Non-Syndicated Episodes?

A: Syndicated eps of Seinfeld are cut. Which means parts of the show are taken out in order to make room for more commercials.
Non-Syndicated eps are the original airing of the show as it was shown the first time.

Q: How is it cut? Are we talking just some actor reactions?

A: No, in some instances an entire Opening Monologue has been cut and the closing Monologue used at the beginning. A perfect example of this is The Bris.